A device as referred to in the introduction is described in European application EP 1 312 547 A1 as forming part of a packaging line for a flowable material, such as cement. The compacting space thereof is formed by a container which is provided with a supply opening, which can be sealed gastight by means of a cover, at the upper side and with a discharge opening, which can be closed by means of a pivoted bottom, at the bottom side. Disposed above the supply opening is a metering device, by means of which a metered supply of flowable material to the container through the open supply opening can be effected with the discharge opening in its closed condition. After the container has been filled with flowable material to a desired extent, the supply opening cover closes, as a result of which the interior of the container is sealed gastight from its environment. A vacuum pump is connected to the container, by means of which air can be extracted from the container, thus creating a reduced pressure in the container. This has a compacting effect on the material in the container of itself already. This effect is enhanced by admitting air, whether or not quickly, to the container again, thus creating a pressure wave which has an additional compacting effect on the material in the container. After the material has thus been compacted, in which connection it is noted that said reduction of the pressure and said admission of air to the container again could also be repeated a few times in succession, the discharge opening is opened, after which the compacted material falls into a package, for example a bag, which is subjected to further processing.
An important problem that occurs when compacting flowable solid material is that usually dust formation takes place to some extent. Such dust is harmful for the vacuum pump that is used for reducing the pressure in the container in which the material to be compacted is present. Consequently it is necessary to use a filter system between the container and the vacuum pump. To increase the efficiency of such filter systems, it is necessary to use filters, preferably fine-meshed ones, and relatively large filter casings that need to be capable of withstanding the sub-atmospheric pressure that is generated. Consequently, the filter casings must be of relatively heavy construction. In addition, such a filter system reduces the efficiency with which the vacuum pump can effect the pressure reduction in the container, as a consequence of which it is necessary to use a vacuum pump of heavier construction as well. Another important drawback related to the use of filters systems is the fact that the dust filters used therein require a great deal of maintenance and need to be exchanged frequently. Usually it is necessary to clean the filter system very frequently, in some cases after every cycle, e.g. by using compressed air, knocking and/or vibrating. Said cleaning steps have a negative effect on the time during which a vacuum pump and the filter system can actually be operational, and thus on the cycle time.
It is noted that U.S. Pat. No. 3,260,285 describes an apparatus and a method for filling containers for pulverulent material, wherein use is made of a combination of a hopper, to the bottom side of which a chute section, a flow control valve and a filler including an upper magazine and a lower flow control head successively connect. In use, a container to be filled is connected gastight to the lower end of the flow control head. The inner side of the flow control valve is provided with a circumferential liner of a flexible material, on the outer side of which a space to be pressurized is present, which makes it possible to force the liner inwards so as to close the valve. The flow control head comprises a valve member, likewise made of a flexible material, which can be inflated inwardly so as to create a bottom for the filler that is to be filled. After a container has been connected gastight to the flow control head, the pressure outside the circumferential liner of the valve member is reduced, as a result of which the valve member will open and the pulverulent material that was present on the valve member functioning as a bottom will fall into the container, aided by a reduced pressure that is generated in the container via a vacuum line. In which a dust filter is mounted. Subsequently, the dust filter is cleared of dust again by pressure blowing.